The invention generally relates to belts used in different phases of a papermaking process. More particularly, the invention relates to shoe press belts used in shoe presses of press sections in board, paper and pulp machines, as well as in shoe presses of a paper machine calender when manufacturing certain types of paper.
Shoe presses are generally used in pulp, cardboard and paper machines for dewatering a fibre web. One surface of a wet fibre web travelling at a high speed in such shoe presses is pressed by a press roll while simultaneously the other surface of the fibre web is pressed by a pressure shoe surrounded by an endless belt with an elastic elastomer body. One such press belt is described in Finnish Patent Application No. 20055556.
In general, a shoe press belt is subjected to recurrent strong flexing and pressing forces between a press roll and a pressure shoe as the fibre web passes at a high speed through the shoe presses. During use, these stresses may cause the belt material to crack over time and, eventually, the belt is damaged beyond usability. A general trend has been to increase both the web speed and the pressing pressure in paper machines. Thus, the belts of the shoe presses used therein are also subjected to larger and larger loads, so the belts are required to possess a plurality of various properties in order to ensure high performance. As the speeds of paper machines increase, a high thermal conductivity of a belt plays an essential role as far as the operating life of the belt is concerned. A high thermal conductivity allows heat to be conducted to the surrounding water and oil, which diminishes the problems caused by excessive heat on the belt, thus extending the operating life of the belt.
EP 1338696 A1 describes a belt suitable for papermaking, comprising a reinforcing substrate embedded in a polyurethane layer. The outer peripheral surface of the belt, which is in direct contact with a supporting press felt of the fibre web, is formed from polyurethane made from a urethane prepolymer and dimethylthiotoluenediamine (DMTDA) as a hardener. The belt aims at decreasing cracking, which generally occurs on outer surfaces of the belts in particular, as well as at preventing delamination between the hardener and the polyurethane layer.
WO 2005/090429 discloses a belt which is coated with polyurethane and which comprises nanoparticles. Such a belt aims at improving e.g. its resistance to flex fatigue and crack propagation as well as at providing hardness and wear characteristics. The nanoparticles are dispersed either in a hardener or in a urethane prepolymer prior to mixing the hardener and the prepolymer so as to obtain a polyurethane. Irrespective of the manner of adding the nanoparticles, a coating contains nanoparticles in a dispersed form. The dispersion of the nanoparticles provides the polyurethane belt with abrasion resistance but, irrevocably, simultaneously impairs the elasticity characteristic of polyurethane. Such impaired elasticity means that the belt becomes more inclined to cracking.
WO 2006/040398 describes hybride nanomaterials which are produced by attaching nanoparticles with different chemical groups. Such attachment is carried out by cutting the nanoparticles by means of ultrasound, whereby highly reactive broken bonds created during the cutting react with the different chemical groups that are present. The nanoparticles consist of substances containing carbon, such as carbon nanotubes. The chemical groups may be inorganic, organic, polymeric and biological molecules and particles. The nanoparticles are capable of bonding with the chemical groups through covalent and non-covalent bonds. The obtained hybrid materials are reported to exhibit a good tensile strength and a high electrical and thermal conductivity, and they are considered suitable for many different uses, such as paper machine rolls and support structures.
It is thus desirable to provide a shoe press belt which is thermally highly conductive and highly resistant to cracking, or, should cracking occur, crack propagation is decelerated substantially.